1. The Field of the Invention
The invention relates to liquid metal electrolyte systems and is especially, though not exclusively, applicable to improving the efficiency and reducing the operating costs of modern-day aluminium reduction cells.
2. The Relevant Technology
The invention will be exemplified, and will subsequently be described and illustrated in this present specification, with reference to aluminium reduction or smelting.
Modern aluminium production plants consume huge amounts of electricity.
Virtually all of them operate by reducing alumina in electrolysis cells or, as they are called, pots. In practise a commercial aluminium smelting plant will consist of several hundred such pots and will operate on a continuous production basis.
There are two remarkable features of this process. First, it has remained virtually unchanged for over a century since it was first successfully developed (and indeed it is still universally known as the Hall-Héroult process after the two scientists who first independently discovered it). Second, the amount of energy consumed by the process is quite staggering.
It has been estimated that the modern-day production of aluminium consumes about two percent of all electricity generated worldwide (!) and yet much of this energy is absorbed in overcoming resistive losses in the poorly conductive highly resistive electrolyte layer of each individual smelting cell. The primary electrical driving current can be of low voltage but must be of relatively enormous amperage in order for the process to work, given these drawbacks, and it follows that any modification which enables that current, the electrolyte thickness, or both, to be reduced at all would indeed produce reductions in energy consumption which could truly be described as significant in relation to those needed without the modification today.
Efforts have been made, naturally, to overcome this problem but the main limiting factor is that, if the electrolyte thickness is reduced beyond a certain critical level, instabilities begin to occur at the interface between the liquid electrolyte and the liquid aluminium. These instabilities, which manifest themselves as a sloshing of the liquids within the cell, have been the subject of intensive research for some 20 years or more. In effect, these are interfacial gravity waves, modified by the external magnetic fields which pervade the cell and when a certain stability threshold is exceeded, these waves can grow by absorbing energy from the ambient electric and magnetic fields.
The good news is that the wave period is measured in minutes and its growth rate in hours, and so the problem ought to be susceptible to some controlled solution. The real problem is that once such a wave takes hold, it can disrupt the electrolysis to such an extent that the cell must be withdrawn from operation. In an extreme case, it could destroy the entire cell.
Previously proposed means for trying to eliminate these instabilities include:                Placing baffles in the aluminium to break up the long-wavelength waves whilst relying on friction to dissipate the short-wavelength components.        Have a sloping cathode block so that the aluminium continually drains away.        Destroy the standing waves by placing hydraulic energy absorbers at the edges of the cell.        Tilt the anode in harmony with the wave so that the electrolyte layer remains almost uniform and thus one eliminates the perturbation in current.        
The first of these prior proposals remains simplistically attractive but both it and the second one are limited by the need to find, in practical environments, a material which survives the chemically aggressive environment in a smelting cell. The second option has another difficulty in that thin aluminium layers will not properly wet the cathode and this cannot easily or cheaply be overcome. Whilst the third option is self-explanatory, the most recent research has concentrated on the final one but as far as is known, no practical embodiment has yet emerged.
Furthermore, a paper published by Elsevier Science dated 12 Nov. 2001 by authors A. Lukyanov, G. El and S. Molokov, is deemed to be relevant to the present application as it defines the general background of the mechanism of instability, however primarily in the context of determining the reflection coefficient rather than proposing a practical solution for controlling instability in a cell as is one of the objectives of the present application.
In summary, despite the length of time the problem has been around, and the importance of modern aluminium production to the progress of industrialised society as a whole in an era when, paradoxically, conservation of energy is becoming more urgent than ever, instability of aluminium reduction cells remains the central and unsolved problem in the industry at large.
The Inventive Concept
The applicants are proposing a modification of existing current-driven liquid metal electrolyte systems (of which an aluminium reduction cell is the obvious but not limiting example) which starts from a point quite different from any of those outlined above—but which could, we believe, be used in any appropriate combination with some, all, or any of the prior proposals outlined above.
In essence, we impose on such a system an additional, external, magnetic field whose design and operating parameters are so chosen as to enable the electrolyte thickness to be reduced significantly in relation to those needed without the modification. By doing this, we address the very source of the instability, which happens due to the interaction of the currents induced by the interface motion with the external magnetic field.
Based on our understanding of the fundamental mechanism governing the instability, we believe it to be possible that, with appropriately designed coils, a ring current around the cell inducing an automating magnetic field will stabilise the cell to an appreciable if not total extent.
Thus, rather than trying to understand fully all the processes happening inside the cell we effectively suppress the fluctuations by imposing a suitably powerful and time-dependent magnetic field around it. A modern aluminium (or any other metal) reduction cell is a complex and highly optimised device. There are a multitude of supple physical and chemical processes occurring within such a cell and many of them will inevitably interact. A small change in any one parameter could well have quite unexpected consequences and these may or may not be either inter-related or predictable at all. The size alone of the primary driving current makes it almost impractical to try to make relatively small adjustments to any one aspect of cell operation—for example, the “anode-tilt” approach exemplified in the fourth prior proposal outlined previously—with any real guarantee of even partial success.
We by contrast take an overview and we believe that, with appropriate design and with the ability to adjust the controlling parameters (i.e. field amplitude, frequency, and constant background) we are more likely to achieve real suppression of instability in a practical format within the foreseeable future.
In a subsidiary aspect, the magnetic field applied is essentially a vertical magnetic field. In this direction, significant effect on instability in liquid metal electrolyte occurs which allows the thickness of electrolyte itself to be reduced below levels at which conventionally instability would occur.
In a further subsidiary aspect, the magnetic field is dependent on an amplitude and frequency whose values are approximated through wave reflection analysis on an infinite wall. This is advantageous as it allows an appropriate magnetic field to be rapidly determined rather than relying on the skilled man to determine an adequate field through more extensive analysis.